There is known in the prior art various types of drive systems for propelling a vehicle. These systems include, for example, internal combustion engines utilizing mechanical transmissions, electric drive systems and hydraulic drive or hydraulic systems. The latter type of system typically includes an internal combustion engine for supplying the power to drive a hydraulic pump which displaces pressurized hydraulic drive fluid to the vehicle wheel motors, commonly referred to as actuators, through various control valves and fluid conduits. Hydraulic drive systems have evolved into sophisticated and often complex arrangements and have found acceptance as a drive system in vehicles used in specialty areas such as construction, agricultural and industrial applications. While hydraulic drive systems function in an acceptable manner, they are not particularly suited for the highly competitive mass-produced passenger vehicle market because of their extremely high cost, complexity and operating drawbacks.
Where hydraulic systems are used to power vehicles, each wheel used to drive the vehicle is rotatably connected to an actuator. Each actuator furnishes the torque necessary to overcome the frictional engagement of the ground with its respective wheel. The pressure drop across each actuator is a function of the torque required by that actuator. The system operates correctly when each tire exhibits similar or identical loads.
In the basic hydrostatic and hydraulic power system, the pressure drop across each motor is equal to the pressure difference delivered by the pump. When all drivable wheels are engaged on similar ground, the load required of each associated drive motor is equal. Therefore, the flow rates to each of the motors and the pressure drops across each of the motors are identical. In the situation just described, the system operates correctly.
Operating problems can occur when not all wheels are in frictional engagement with ground surfaces having the same coefficient of friction. Wheels in contact with a ground surface having a lower coefficient of friction will begin to slip, causing more flow to be directed through the reduced load motor, which in turn reduces the displacement pump pressure and overall efficiency of the hydrostatic system, since a disproportionate amount of the hydraulic flow will take the path of least resistance and flow through the reduced-load actuator.
An example will illustrate. In a four-wheel drive system with an actuator at each wheel, a drive wheel contacting a ground surface having a low friction coefficient such as mud or ice, will slip and spin faster than the other wheels due to a reduced load acting on the actuator connected to that wheel. This results in a reduced pressure drop across that actuator which causes a system wide pressure drop where a larger than necessary portion of the pump displacement flow is directed to the slipping actuator than to the other load-bearing actuators which in fact would need the flow in order to maintain wheel rotation at a desired rate. In the worst case, the resultant system-wide flow pressure is insufficient to provide the necessary traction force for the load bearing wheels to overcome the frictional contact with the ground, and they stall.
In many powered systems there is a need for on-off controls that will either energize a particular part of the system, or cut off the flow of energy to it. If there is a multi-branch system and trouble develops with a particular branch, the remedy would often be to simply shut off power to that branch. In a vehicle drive system where there is a problem of wheel slippage, however, it is convenient to refer to the problem as being the partial loss of a load, rather than a complete loss. In other words, the load is still there and must still receive energy, but the proper operating relationship between fluid pressure and fluid flow rate being supplied to the wheel motor, and countertorque developed by the wheel, has been disrupted. It is therefore desirable to restore the proper balance as quickly as possible.
The present applicant has provided an overview addressing operating problems and the available componentry to control and improve performance of hydraulically powered vehicles in an article published in Hydraulics & Pneumatics, October 1988. The article was entitled "Some thoughts on wheel drives and traction controls for mobile equipment", by Robert B. Lauck.
More recently, an improvement developed by Poclain Hydraulics of France has focused on a system wherein speed sensors are fitted at the motors to detect the rotational speed at each wheel. Each speed signal is transmitted to an electronic card for comparison of wheel speeds. When the rotational speeds between the wheels exceed an allowed rate, the electric card will transmit a signal to a linked solenoid valve associated with the faster rotating or slipping wheel for controlling a spool, which creates a pressure drop on the supply to the slipping wheel, which will reduce its spin rate. In the Poclain system the solenoid valve is actuated by fluid pressure from a line that is referenced to the tank pressure.